The overall objectives of this project are the synthesis and characterization, by analytical, spectroscopic and crystallographic methods, of transition-metal complexes of organic disulfides, R-C-S-S-C-R. Relationships between structure, spectra and chemical properties are noted and utilized in the description of these systems. On this basis, the in vivo role of such complexation in metalloproteins or otherwise can be better understood and more readily discovered. The facile oxidation of mercaptides and reduction of disulfides, coupled with the ability of sulfur in either the 0 or -1 oxidation state to complex strongly with metal ions which can also change oxidation state, has created a class of oxidation-reduction functions which are likely to be widely utilized by metalloproteins. This expectation has been fulfilled for the metal-sulfur proteins ferredoxin, rubredoxin and caeruloplasmin. An active-site in caeruloplasmin has been identified, involving two Cu(I) ions and a coordinated aliphatic disulfide group, which is very similar, according to the uv and laser-Raman spectra, to that observed in the two Cu(I) disulfide complexes which we had prepared and determined the structures of. Complexes of Cu(I) with C-S-S-C show modified S-S spectra, very short Cu(I)-S distances, and distorted dihedral angles. Complexes of Ni(II), in contrast, show none of these modifications, except for steric reasons. Other copper oxidases, two electron transfer agents, may function by coordinated disulfide mechanisms. Complexes of Fe(II), Fe(III), Co(II), Pd(II), Zn(II), Cd(II), Ni(II), (Cu(I), Mn(II), and Hg(II) will be prepared, if possible, characterized, and studied crystallographically and spectroscopically. These complexes can guide the notions of biochemists toward the plausible but unusual structures which are likely to exist. When an identification is made, these model structures give a detailed glimpse of the protein active site.